The estimation of bubble populations in the surf-zone by inversion of acoustic propagation

Abstract

For several decades the propagation characteristics of acoustic pulses (attenuation and sound speed) have been inverted in attempts to measure the size distributions of gas bubbles in liquids. Primarily this has been attempted in the ocean for defence and environmental purposes, however there are a growing number of biomedical and industrial applications. In order to simplify the inversion, previous investigators have assumed that that the bubbles are undergoing linear. steady-state monochromatic pulsations in a free field, without interacting. These assumptions are always contravened to some extent. This study examines the validity of the assumptions and identifies the need for a new time-dependent nonlinear method of determining a bubble's extinction cross section. Such a model is developed and employed in an experiment to estimate the bubble population in the surf-zone, an important but seldom measured region of the ocean. where large populations of bubbles are generated by breaking waves. The necessary theoretical framework to exploit this new model (based on the current state-of-the-art technique) is developed and employs a new method of determining the optimal regularisation parameter for use in the inversion process. A series of laboratory tests and surf-zone sea trials are described that result in a set of bubble populations calculated using linear and, for the first time. nonlinear techniques

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